Blackleg
Pectobacterium atrosepticum
also called potato blackleg
A bacterial stem rot of potatoes: shoots are stunted and yellow, and the stem base turns inky black and slimy from the seed tuber up.
What you'll see
- The base of the stem turns black, soft and slimy, rotting upwards from the seed tuber — the sign that names it.
- Shoots come up stunted, stiff and upright, with yellowing leaves that roll and curl.
- Affected plants wilt and collapse, and pull away easily from the rotten black stem base.
- The seed tuber below has rotted to a wet, foul-smelling pulp.
- Daughter tubers may rot at the heel end where they join the stolon.
Why it happens
Bacteria that rot the stem and tubers, usually carried in on infected seed potatoes. The rot spreads from a diseased seed tuber up into the young stem, and bacteria wash through wet soil to nearby plants.
Cold wet soil at planting and a wet season bring it on; waterlogged ground and warm humid weather afterwards make it worse.
Treating it
- Lift and destroy affected plants along with the rotten seed tuber as soon as blackleg shows.
- Do not compost infected material, and lift neighboring plants carefully.
- Harvest a lightly affected crop in dry conditions and use it soon, as it will not store.
- Sort out and discard any soft or rotten tubers before storing.
Keeping it away
- Plant only certified, healthy seed potatoes and reject any that are soft or rotten.
- Wait for the soil to warm and drain before planting, and avoid waterlogged ground.
- Rotate potatoes around the plot and never save seed from an affected crop.
- Handle seed tubers gently to avoid bruising, and let cut or chitted seed heal before planting.
- Lift the crop in dry weather and store only sound, dry tubers.
Who gets it
Potatoes above all; the same bacteria cause soft rots in a wide range of other crops.
See also
The Cottage Garden Companion — the gardener's apothecary at cottagegardencompanion.com